Cost not change at second call BAPI_SALESDOCU_CREATEFROMDATA after MIRO

The business flow is to create dummy return sales order to flush of the stock in order to do credit adjustment. and after adjustment to do dummy sales order to counteract return.

Return SO->MIRO->SO

Prgoram steps as below:

1.BAPI_SALESDOCU_CREATEFROMDATA (create return so)

2.WS_DELIVERY_UPDATE

3.BAPI_BILLINGDOC_CREATEMULTIPLE (billing 1)

4.BAPI_INCOMINGINVOICE_CREATE

5.BAPI_SALESDOCU_CREATEFROMDATA (create so)

6.WS_DELIVERY_UPDATE

7.BAPI_BILLINGDOC_CREATEMULTIPLE (billing 2)

But when the so is created. the cost of so is still the old one (same as return). but actually in MBEW, the cost is new.

However the cost of billing 2 is the new one.

it is very strange that the cost of so is not same as billing 2.

and why the cost of so is not changed?

whether i should clear the buffer when finished step 4?

how can I do?

regards,